Apparatus and method for producing a solid magnet roller using a movable mold

ABSTRACT

An apparatus and a method for manufacturing a magnet roller makes it possible to remarkably reduce defects on the surface or the inside of a magnet roller and also to control “warp” of the magnet roller sufficiently so that it does not adversely affect the functions of the magnet roller. A metal mold for magnetic field injection molding is composed of two fixed mold counterparts and a movable mold counterpart. The movable mold counterpart is moved to increase the volume of the cavity of the metal mold as a resin-bonded magnet material is injected into the mold.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus and a method forproducing a magnet roller which is incorporated in a developing roller,a cleaning roller, a toner carrying roller, or the like used for anelectrophotographic copier, a laser beam printer, a facsimile, or thelike.

[0003] 2. Description of the Related Art

[0004] An electrophotographic apparatus or an electrostatic recordingapparatus such as a copier and a laser beam printer is designed to makea magnetic developing agent, namely, toner, which is carried by adeveloping roller, adhere to an electrostatic latent image formed on animage bearing member such as a photosensitive drum, thereby developingthe image. The developing roller has a magnet roller which is formedusing resin-bonded magnet and which is disposed in a rotatingnonmagnetic sleeve; it carries the magnetic toner to a vicinity of thesurface of the photosensitive drum in such a manner that the magnetictoner is shaped like a spike or like a thin layer on the surface of thesleeve, then it develops an image by making the toner adhere to theelectrostatic latent image by bringing the toner in contact with thephotosensitive drum or by jumping development.

[0005] Hitherto, the aforesaid magnet roller is produced by shaping apelletized resin magnet composition, which is composed of athermoplastic resin binder such as nylon and polypropylene with magneticpowder such as ferrite mixed therein, by injection molding or extrusionmolding using a metal mold with a magnetic field generator disposedaround the metal mold, thereby magnetizing the roller to a desiredmagnetic characteristic. In this case, the roller is usually providedwith shafts on both ends thereof; a single shaft may be fed through theroller in the axial direction to provide the shafts on both ends,separate shafts may be provided on both ends, or the shafts on both endsor a shaft on one end is molded integrally with a roller body by using acomposition for resin-bonded magnet.

[0006] The manufacturing method for the magnet roller whereby the metalmold with the magnetic field generator disposed therearound forinjection molding is called a magnetic field injection molding method.According to the magnetic field injection molding method, a meltedresin-bonded magnet material composed primarily of resin powder and abinder is injected into the cavity of a two-piece metal mold, then amagnetic Field is applied from outside around the cavity of the metalmold to magnetize it, thereby producing resin magnet. Then, the metalmold is cooled to harden the magnet roller made of the resin magnet andthe mold is split to take out the completed magnet roller.

[0007] Referring to FIG. 14, the manufacturing method for a magnetroller which is based on the conventional magnetic field injectionmolding method will be described. FIG. 14 is a cross-sectional view of aresin-bonded material which has been injected in a magnetic fieldinjection molding metal mold 100 constituted by cover mold counterparts101 and 102, dwell pressure being applied thereto. A cavity 104 for themagnet roller is formed by the cover mold counterparts 101 and 102,approximately half thereof being in the respective cover moldcounterparts. The mold temperature is maintained at a fixed level by acooling pipe, not shown. One of the cover mold counterpart 101 or 102 ismounted, as a fixed mold, on the resin-bonded magnet material injectionmolding apparatus, not shown, and the other is mounted as a movablemold. A magnetic field generator 106 is provided in the vicinity of themetal mold 100. The injection molding apparatus is equipped with anozzle 105.

[0008] As shown in FIG. 14, to mold the magnet roller by the magneticfield injection molding method, the metal mold 100 is formed by clampingthe fixed mold counterpart and the movable mold counterpart with aparting line 107 as the boundary, a melted resin-bonded magnet material10 is injected through an injection hole 102A while applying a magneticfield to the cavity 104 of the metal mold 100 by the magnetic fieldgenerator 106, thereby magnetizing the resin-bonded magnet material 10to mold the resin magnet. After cooling time elapses, the metal mold 130is released and the magnet roller made of the resin magnet is taken out.

[0009] The magnet rollers manufactured according to the conventionalmagnetic field injection molding method, however, have such defects asflow marks on the surfaces thereof or bubbles at the central portionsthereof from time to time.

[0010]FIG. 15 is a typical explanatory diagram showing the flow marksand bubbles that take place in injection moldings. These defects arelikely to occur when a melted resin with high viscosity is used forinjection molding. When the viscosity of the melted resin is high,melted resin 111 is shaped like a string in the early stage of fillingwhen it is injected from the gate into a metal mold 110 as illustratedin FIG. 15 (A). As is schematically shown in FIG. 15(B), the meltedresin, which has been injected in the string-like shape, ends up beingcharged in the cavity in a compressed state. Melted resin 112, which hasbeen shaped like entangled strings, is compressed and fused, leading tothe formation of a fused mark 113 which is known as the flow mark.Further, as schematically illustrated in FIG. 15(A), when thestring-like melted resin is injected into the metal mold, air in thecavity is sometimes caught in a spot 115 between the entangled meltedresin strings. The result is a bubble 116 in the finished molding asshown by the cross section of FIG. 15(C). This is the defect known asthe bubble inside.

[0011] Even if magnet rollers are free of such defects, they tend tohave warp in the lengthwise direction more or less, whereas they shouldbe straight in the axial direction. If a magnet roller is warped, thenthe gap between the surface of the roller and the sleeve, which isformed when a nonmagnetic sleeve is attached, varies according to thelengthwise position of the roller. As a result, the developing roller inwhich the warped magnet roller has been incorporated will have amagnetic force characteristic which varies in the axial direction of theroller, causing unstable toner carrying function. The magnetic pole ofthe warped magnet roller is frequently twisted in the lengthwisedirection.

[0012] Thus, the magnet rollers manufactured according to theconventional method have many defects and incur warp more or less,leading to the need for inspection on every magnet roller for warp inthe production process. This calls for inspection equipment andinspection man-hours; those magnet rollers, the warp of which exceeds atolerance, are eliminated as defective products.

SUMMARY OF THE INVENTION

[0013] The present invention has been accomplished with a view towardsolving the problem with the prior art stated above, and it is an objectof the present invention to provide an apparatus and a method forproducing a magnet roller which make it possible to remarkably decreasethe defects on the surfaces and/or inside of magnet rollers and tosecurely obtain magnet rollers in which the warp thereof has beencontrolled to such an extent that it does not lead to functionalproblems.

[0014] The inventors have repeatedly carried out various experiments toidentify the cause of the bubbles in the core of a magnet roller whichhas been manufactured using the conventional magnetic field injectionmolding method, or of such defects as a flow mark on the surfacethereof, or the warp thereof. The inventors have found that theseproblems occur because the melted resin-bonded magnet material injectedinto the mold is free to flow in the cavity of the metal mold and itstarts to settle and accumulate first in an area where the magneticforce is the strongest.

[0015] More specifically, when the melted resin-bonded magnet materialis injected under high pressure into the cavity of the metal mold towhich the magnetic field is being applied, the melted resin-bondedmagnet material, which has been poured through the injection bole,starts to move around in the cavity of the mold; however, the magneticfield causes the melted resin-bonded magnet material to begin adheringand accumulating first onto the cavity wall surface which has strongermagnetic force. At this point, because the intensity of the magneticfield in the cavity changes in the circumferential direction, theadherence and accumulation of the resin-bonded magnet material onto thecavity wall surface also changes in the circumferential direction.

[0016] Continuing the injection of the melted resin-bonded magnetmaterial causes the melted resin-bonded magnet material to partiallyaccumulate and also causes a part thereof on the cavity wall surface tosolidify due to the cooling effect of the mold. This causes theresin-bonded magnet material to be deposited and formed, beginning fromthe outer circumferential surface and then toward the axis of the magnetroller. This means that the resin-bonded magnet material is not chargeduniformly in weight into the magnet roller cavity and the molecularorientation of the binder is nonuniform. As a result, such defects asbubbles occur at the central area of the magnet roller or such defectsas the flow marks are produced on the outer surface.

[0017] The charging process of the resin-bonded magnet material statedabove is also responsible for uneven circumferential heat history of theresin-bonded magnet material on the outer surface of the magneticroller, which in turn leads to the uneven circumferential shrinkage ofthe outer surface of the magnet roller. The uneven shrinkage is partlyresponsible for the lengthwise warp of the magnet roller.

[0018] The problem with the conventional magnetic field infectionmolding method is attributable to the fact that the melted resin-bondedmagnet material which has been injected into the cavity of the mold isallowed to move freely under the influences of the magnetic forces. Itis therefore considered to be possible to control the chance of theoccurrence of the defects at the central part as well as on the surfaceof the magnet roller and also of the warp by filling the cavity of themetal mold with the resin-bonded magnet material uniformly and densely.

[0019] Furthermore, the inventors have studied magnet rollers withdifferent structures in-relation to the above-described problems causedby the conventional techniques. As a result, they found that in the casewhere a resin-bonded magnet material is injected in a metal moldinserted with a shaft therethrough, the injected material is more apt toform a bulk since the material is charged while making contact with theshaft (this phenomenon is notable when the shaft has a diameter of A anda magnet body of the magnet roller has a diameter of 4A or less), andsuch defect as warp is less likely to occur due to the presence of theshaft. On the other hand, the inventors recognized that in the casewhere a resin-bonded magnet material is injected in a metal mold withoutinserting the shaft therethrough such that a solid magnet body ismolded, the above-described problems took place mainly because theresin-bonded magnet material was charged into the metal mold asillustrated in FIG. 15(A) through FIG. 15(C).

[0020] The magnet roller with the shaft, however, is associated withsuch problems as an increase in the cost for the use of the shaft, andan increase in the number of steps for pre-inserting the shaft throughthe metal mold in manufacturing the magnet roller.

[0021] The present invention has been accomplished by such awareness ofthe problem with the conventional manufacturing method for the magnetrollers, especially magnet rollers with solid bodies, and also by theefforts made for identifying the cause; and the objects thereof areachieved by making the volume of the mold cavity variable andrestricting the free movement of the resin-bonded magnet material in thecavity, thereby enabling uniform and dense charging of the resin-bondedmagnet material into the cavity.

[0022] Specifically, an apparatus for producing a magnet rolleraccording to the present invention includes: a fixed mold having acavity for forming a solid magnet body; a movable mold disposed in thecavity, which is capable of increasing or decreasing a volume of thecavity in accordance with an injected amount of the melted resin-bondedmagnet material composed primarily of magnetic powder and a binder; anda magnetic field generator disposed around the fixed mold.

[0023] After starting the injection of the resin-bonded magnet material,a movable mold is moved in the direction for increasing the volume ofthe cavity as the injection of the resin-bonded magnet material iscontinued.

[0024] Starting the injection of the resin-bonded magnet material whenthe volume of the mold cavity is the smallest prevents the first flow ofthe resin-bonded magnet material from dispersing in the cavity andallows the resin-bonded magnet material to be charged so that the crosssection of the cavity is fully charged from the injection hole side.

[0025] The movable mold is moved away from the injection hole of themetal mold while the resin-bonded magnet material is being charged. Themovable mold is moved by the flowing pressure of the injectedresin-bonded magnet material. At this point, providing a biasing forceto the movable mold in the direction for decreasing the volume of thecavity by biasing means which in turn is provided with the biasing forceby an air cylinder, a coil spring or the like, prevents the volume fromexceeding the injection amount of the resin-bonded magnet material.Furthermore, when the movable mold moves along the mold wall surfaceunder the flowing pressure of the resin-bonded magnet material,supporting the movable mold by the biasing means as the magnetic resinmaterial is injected makes it possible to maintain a proper balancebetween the injection amount of the resin-bonded magnet material and theincrease in the volume of the cavity. The biasing means extends in thecavity to thereby provide a biasing force to the movable mold. Themovable mold moves in the lengthwise direction of the cavity against thebiasing force provided by the biasing means extending in the cavity,preferably with a back pressure of 0.5 to 50 kg/cm².

[0026] A method for producing a magnet roller according to the presentinvention is a method in which a resin-bonded magnet material, which iscomposed primarily of magnetic powder and a binder, is injected into acavity of a metal mold while applying a magnetic field thereto. A fixedmold having a cavity for forming a solid magnet body and a movable molddisposed in the cavity and capable of increasing or decreasing a volumeof the cavity are used as the metal mold. While injecting a meltedresin-bonded magnet material into the cavity, the movable mold is movedsuch that the volume of the cavity is increased in accordance with theinjected amount of the melted resin-bonded magnet material, and a magnetbody molded within the cavity is magnetized by a magnetic fieldgenerator disposed around the fixed mold.

[0027] The movable mold moves in a lengthwise direction of the cavitypreferably with a back pressure of 0.5 to 50 kg/cm² against a biasingforce provided by biasing means extending in the cavity.

[0028] According to the present invention, the volume of the mold cavityincreases only by the volume corresponding to the increase in the amountof the injected resin-bonded magnet material; therefore, theresin-bonded magnet material can be uniformly and densely injected allthe way to the center of the mold cavity. This prevents the core of thecompleted magnet roller from incurring the bubbles or other defects andalso prevents the surface thereof from incurring the flow marks or otherdefects.

[0029] Further, the heat history of the resin-bonded magnet material onthe outer surface of the magnet roller becomes uniform in thecircumferential direction, resulting in uniform shrinkage in thecircumferential direction. This minimizes the chance of the magnetroller from warping in the lengthwise direction.

[0030] The surface roughness of the magnet roller produced according tothe present invention is 20 mm or less based on a Japanese IndustryStandard (JIS) 10-point average roughness scale Rz (JIS B0601-1982).When the magnetic force of the magnet roller is measured at intervals of1 mm in the direction parallel to the axis thereof, the difference inmagnetic force between the adjacent points is 10 gauss or less. Themagnet roller which has such a small surface roughness value andexhibits the uniform magnetic property along the axis thereof cannot beproduced by the conventional manufacturing method; it was not until thepresent invention was accomplished that the magnet roller having thedesirable characteristics stated above became available.

[0031] The term surface roughness Rz used herein is a ten-point meansurface roughness by JIS B0601-1982. The terms, surface roughness,profile, reference length of profile, roughness curve, cut-off value,mean line of profile, and profile peak and valley are as defined in thestandard. In FIG. 16, the ten-point mean roughness shall be the value ofdifference, being expressed in micrometer (mm), between the mean valueof altitudes of peaks from the highest to the 5th, measured in thedirection of vertical magnification from a straight line a that isparallel to the mean line and that does not intersect the profile, andthe mean value of altitudes of valleys from the deepest to the 5th,within a sampled portion, of which length corresponds to the referencelength, from the profile. The profile may be depicted by means of aprobe meter, for example. The ten-point mean roughness Rz is given bythe following equation:

Rz=[(R 1+R 3+R 5+R 7+R 9)−(R 2+R 4+R 6+R 8+R 10)]/5

[0032] wherein R1, R3, R5 R7 and R9 are altitudes of peaks from thehighest to the 5th for the sampled portion corresponding to thereference length L, and R2, R4, R6, R8, and R10 are altitudes of valleysfrom the deepest to the 5th for the samples portion corresponding to thereference length L. The reference length L varies with the range of theten-point mean roughness Rz and it is also in conformity to thestandard. For example, L=0.25 mm when Rz<0.8 mm, L=0.8 mm when 0.8mm<Rz<6.3 mm, L=2.5 mm when 6.3 mm<Rz<25 mm, and so on.

[0033] The magnet roller is capable of generating uniform magnetic forcein the axial direction without the need for modifying the externaldimensions of the magnet roller by grinding, cutting, or other type ofmachining. Hence, using the magnet roller as the developing roller for adeveloping apparatus enables the formation of good images.

[0034] The magnet roller produced according to the present invention isideally used as the magnet roller constituting the developing roller,the cleaning roller, or the toner supplying roller of a copier, aprinter, or other electrophotographic apparatus or an electrostaticrecording apparatus.

[0035] For example, the magnet roller produced according to the presentinvention is best suited for the developing roller or the developingagent supplying roller in a developing apparatus which develops anelectrostatic latent image, which has been formed on an image bearingmember, by using a developing agent so as to visualize the image. Thedeveloping agent supplying roller is disposed together with thedeveloping roller in the developing apparatus; it supplies toner to thedeveloping roller.

[0036] The magnet roller produced according to the present invention issuitably used as a magnet roller for use in a cleaning apparatus forremoving a developing agent which remains on an image carrier after anelectrostatic latent image formed on the image carrier is transferred toa transfer material as a visible image by the developing agent. In thecleaning apparatus, a cleaning roller incorporating a magnetic roller isused to collect toner by magnetic force after the toner remaining on theimage carrier such as a photosensitive drum is scraped off with acleaning blade. In this case, the magnetic roller is arranged at aposition suitable for the collection of the toner, the toner is adsorbedto the magnetic roller by magnetic force, and the toner is scraped offfrom the magnetic roller by the blade at a predetermined position sothat the toner is collected by a predetermined toner collection unit.The cleaning roller is also used as a roller for removing tonerremaining on an image carrier by applying magnetic force to the toner orfor facilitating the removal of toner with a cleaning blade.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a cross-sectional view illustrative of an example of ametal mold for magnetic field injection molding used in a method inaccordance with the present invention;

[0038]FIG. 2 is a cross-sectional view of the metal mold for magneticfield injection molding during the application of dwell pressure;

[0039]FIG. 3 is a cross-sectional view of a magnet roller made integralwith a shaft;

[0040]FIG. 4(A) and (B) are cross-sectional views illustrating thestructures of the magnet rollers;

[0041]FIG. 5 is a schematic cross-sectional view of a developing roller;

[0042]FIG. 6 is a cross-sectional view illustrative of another exampleof the metal mold for magnetic field injection molding used in themethod in accordance with the present invention;

[0043]FIG. 7 is a cross-sectional view illustrative of still anotherexample of the metal mold for magnetic field injection molding used inthe method in accordance with the present invention;

[0044]FIG. 8 is a cross-sectional view illustrative of a metal mold formagnetic field injection molding for the comparison;

[0045]FIG. 9 is a cross-sectional view illustrative of the metal moldfor magnetic field injection molding for the comparison, which ismolding the magnet roller;

[0046]FIG. 10 is another cross-sectional view illustrative of the metalmold for magnetic field injection molding for the comparison, which ismolding the magnet roller;

[0047]FIG. 11 is a schematic diagram of a copier;

[0048]FIG. 12 is a schematic explanatory diagram of a developingapparatus;

[0049]FIG. 13 is a schematic explanatory diagram of a cleaningapparatus;

[0050]FIG. 14 is a cross-sectional view of a metal mold which is moldinga conventional magnet roller; and

[0051]FIG. 15(A) to FIG. (C) are explanatory diagrams of a flow mark anda bubble;

[0052]FIG. 16 illustrates a roughness profile for explaining ten-pointmean roughness Rz according to JIS B-0601; and

[0053]FIG. 17 is a cross-sectional view illustrative of the structure ofa magnet roller for the comparison made integral with a shaft.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0054] The preferred embodiments of the present invention are describedhereinafter with reference to the accompanying drawings.

[0055]FIG. 11 is a schematic diagram for explaining an exemplary copierof electrophotographic system comprising the developing apparatusincorporating a magnet roller manufactured according to the presentinvention and a cleaning apparatus. This copier comprises a cylindricalphotosensitive drum 61 which rotates in a direction shown by an arrow,and a primary charging apparatus 62, a light source 63 for imageexposure, a developing apparatus 65, a transfer apparatus 66 and acleaning apparatus 67, all arranged around the photosensitive drum 61.The photosensitive drum 61 is charged with several hundreds of volts bythe charging apparatus 62 and an image of an original illuminated by thelight source 63 is formed at an image exposure section. A surface chargeselectively disappears owing to this image exposure and an electrostaticlatent image is formed on the photosensitive drum 61. The developingapparatus 65 makes toner to adhere to the electrostatic latent imageformed on the photosensitive drum 61 to visualize it. The visible imageformed by the toner is transferred to transfer paper 68 by the transferapparatus 66 which charges the transfer paper 68 with electricity fromthe rear surface thereof. A toner image thus transferred to the transferpaper 68 is fixed to the transfer paper 68 by a fixing apparatus 69. Thetoner remaining on the photosensitive drum after image transfer by thetransfer apparatus 66 is removed by the cleaning apparatus 67 and thephotosensitive drum having a clean surface is charged with apredetermined voltage by the primary charging apparatus 62 again. Thusexposure and development are repeated.

[0056]FIG. 12 is a schematic diagram illustrative of an example of adeveloping apparatus 65. Disposed in housing 75 of the developingapparatus 65 are a toner carrying roller 71 for carrying magnetic tonerfrom a toner chamber, a developing roller 72 for carrying the toner,which has been carried out by the Loner carrying roller 71, to thesurface of a photosensitive drum 61 to attach it to an electrostaticlatent image, and a doctor blade 73 for keeping the thickness of amagnetic toner layer on the developing roller 72 constant. The tonercarrying roller 71 is a sleeveless magnet roller; and the developingroller 72 is a magnet roller provided with a nonmagnetic sleevetherearound. These developing roller 72, the photosensitive drum 61, andthe toner carrying roller 71 respectively rotate in the directions shownby the arrows in the drawing to supply the magnetic toner from the tonercarrying roller 71 to the surface of the developing roller 72. The toneris formed into a thin layer of uniform thickness by the doctor blade 73and the thin layer of the toner is carried onto the surface of thephotosensitive drum 61 whereon the toner adheres to the electrostaticlatent image.

[0057]FIG. 13 is a schematic diagram showing an example of the cleaningapparatus 67. A cleaning blade 92 made of an elastic material such asurethane rubber, a cleaning roller 93 formed of a magnetic roller and adoctor blade 94 for keeping the thickness of a magnetic toner layer 95on the cleaning roller 93 constant are arranged inside the housing 91 ofthe cleaning apparatus 67. The toner remaining on the photosensitivedrum 61 is removed by interaction between it and the magnetic tonerlayer 95 on the cleaning roller 93 or its adhesion to the surface of thephotosensitive drum is weakened. The photosensitive drum 61 is coveredwith a magnetic toner layer by contacting the magnetic toner layer 95.The magnetic toner layer covering the photosensitive drum 61 is removedby the cleaning blade 92 together with the remaining toner. The thusremoved toner is discharged into an unshown collector.

[0058]FIG. 1 and 2 are cross-sectional views of an exemplary mold formagnetic field injection molding which is used in the production of themagnetic roller. FIG. 1 shows the injection operation state of a moldingcycle and FIG. 2 shows the pressure maintenance operation state.

[0059] The mold 20 for magnetic field injection molding consists offixed molds 1 and 2 for forming a cavity 4, and a movable mold 3. Themovable mold 3 is incorporated in the fixed mold 2. The movable mold 3is made of a metal or resin cylindrical body and provided with at leastone O ring 3E made of rubber or the like on the outer surface thereof toprevent the effluence of a melted resin magnetic material. The fixedmolds 1 and 2 are each provided with a magnetic generating apparatus 6on the outer side thereof to apply a magnetic field to all or part ofthe cavity 4. With a parting line 7 serving as a borderline betweenmovable and fixed sections of an unshown injection molding apparatus,the fixed mold 1 is attached to the movable section of the injectionmolding apparatus and the fixed mold 2 to the fixed section of theinjection molding apparatus. A cooling pipe, not shown, for controllingthe temperatures of the molds is built in the injection moldingapparatus so that the temperatures of the mold 1 and the mold 2 arepreferably maintained at 100 to 110° C. An injection hole 2A forinjecting a resin magnetic material, which engages with the nozzle 5 ofthe injection molding apparatus, is provided above the molds 1 and 2.

[0060] Referring to FIG. 1, a movable mold 3 constitutes a part of thewall surface of a cavity 4; it has cavity wall surface segments 4B, 4C,and 4D of the magnet roller. The movable mold 3 is free to verticallyslide on a mold wall surface segment 4E which is constituted by covermold counterparts 1 and 2. The rear end of the movable mold 3 isconnected to a sliding rod 3A which is provided with a force toward aninjection hole 2A by an air cylinder 21. The biasing force given by theair cylinder 21 is set to approximately 5 kgf when the injectionpressure of a resin-bonded magnet material is, for example, 500 kgf. Theback pressure provided by the air cylinder 21 or the like, i.e., theload on the movable mold 3 over a cross-sectional area of the cavity 4,is preferably 0.5 to 50 kg/cm². When the back pressure is less than 0.5kg/cm², a density of the charged resin-bonded magnet material becomeslow, whereas when the back pressure exceeds 50 kg/cm², such problemsarise as a magnetic force of the obtained magnet body is deteriorated.

[0061] The resin-bonded magnet material is prepared by mixing andkneading a binder and magnetic powder and forming the mixture intopellets. For the binder, the following may be employed: nylon 6,polystyrene, polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), PPS, EVA, EEA, EvOH, polypropylene, polyethylene,polyethylene copolymer, or other polyolefin, or a modified polyolefinproduced by introducing a reactive functional group such as a maleicanhydride group, a carboxyl group, a hydroxyl group, and a glycidylgroup into the structure of the polyolefin.

[0062] The blending quantity of the binder is not restricted, however,the binder of about 8 to about 40 wt % is commonly used (most commonly10 to 20 wt %).

[0063] For the magnetic powder used in this embodiment, magnetic powdercommonly used for resin magnet may be employed; examples include bariumferrite, strontium ferrite, or other ferrite, an Sm—Co type alloy, anNd—Fe—B type alloy, or other rare earth type alloy. There is noparticular restrictions on the blending quantity of the magnetic powder;it is properly decided according to the magnetic property required ofthe magnetic roller, however, it commonly ranges from 60 to 92 wt % ofthe entire resin magnet composition, most commonly about 80 to about 90wt %.

[0064] As necessary, a filler with great reinforcing effect such asmica, whisker, talc, carbon fiber, and glass fiber may be added to theresin magnet composition in addition to the binder and magnetic powder.More specifically, if relatively low magnetic force is required of themolding and the charging quantity of the magnetic powder such as ferriteis small, then the rigidity of the resulting molding tends to be low;therefore, such filler as mica or whisker may be added to increase therigidity of the molding.

[0065] In this case, mica or whisker is suitably used as the filler. Forthe whisker, the following may be preferred: a nonoxide based whiskercomposed of silicon carbide, silicon nitride, etc., or a metal oxidebased whisker composed of ZnO, MgO, TiO₂, SnO₂, Al₂O₃, etc., or acomplex oxide based whisker composed of potassium titanate, aluminumborate, chlorinated magnesium sulfate, etc. Among these, the complexoxide based whisker is especially suited for the purpose because of itsgood compatibility with plastics.

[0066] There is no particular restrictions on the blending ratio for thefiller; however, it typically ranges from 2 to 32 wt % of the entireresin magnet composition, most typically about 5 to about 20 wt %.

[0067] The method for filling the cavity 4 with the melted resin-bondedmagnet material while moving the movable mold 3 will now be described.

[0068] When a metal mold 20 for magnetic field infection molding isclamped, the movable mold 3 is disposed toward the injection hole 2Aalong the mold wall surface segment 4E in such a manner that it juts outinto a hollow 14 of the metal mold due to a biasing force provided bythe air cylinder 21 via the sliding rod 3A; the cavity wall surfacesegment 4B of the movable mold 3 is almost in contact with or in contactwith the cavity wall surface segment 4B of the mold counterparts 1 and2. Hence, the volume of the cavity 4 in the state, where the sliding rod3A is provided with a force by the air cylinder 21, is equal to thevolume of the hollow formed by the cavity wall surface segment 4C of thefixed mold counterparts 1 and 2 and the cavity wall surface segments 4Cand 4D of the movable mold 3. This volume is substantially the smallest.

[0069] As shown in FIG. 1, from the state described above, the magneticfield generator 6 is actuated to apply a predetermined magnetic fieldand the melted resin-bonded magnet material 10 is injected through anozzle 5 as indicated by an arrow A into the hollow formed by the cavitywall surface segments 4B, 4C, and 4D of the mold 3. Flowing pressure Fof the melted resin magnetic material 10 is received by the cavity wallsurface segments 4B, 4C, and 4D of the movable mold 3.

[0070] Continued injection of the melted resin-bonded magnet material 10further increases the flowing pressure; in response to the increase inthe flowing pressure, i.e. the increase in the melted material, themovable mold 3 starts to move back in the opposite direction from thedirection of the biasing force, i.e. downward in FIG. 1, by the increasein the volume of the melted material while balancing with the biasingforce of the air cylinder 21 via the sliding rod 3A. Thus, the area ofthe cavity wall surface segment 4A gradually increases. Continuing theinjection of the melted resin and the reverse movement of the movablemold 3 at the same time enables the resin-bonded magnet material 10 tobe uniformly and densely charged in the cavity of the mold as shown inFIG. 2.

[0071] During the molding process, the resin-bonded magnet material 10is magnetized into a resin magnet by the applied magnetic field. Coolingthe mold counterparts 1 and 2 at a constant rate molds a magnet roller12 composed of the resin magnet. After a predetermined time elapses, themetal mold 20 is split to take out the magnet roller 12 made integralwith a shaft which has been molded as shown in FIG. 3. The magnet roller12 shown in FIG. 3 has shafts 26A and 26B on both ends of a magnet body25 which have been molded as one piece with the magnet body 25 by usinga resin magnet composition.

[0072] As shown in FIG. 4(A), the shaft section of the magnet roller maybe composed of metallic shafts 28A and 28B which do not pass through themagnet body 25 but are buried so that they jut out in the axialdirection from both ends of the magnet body 25. Further alternatively,as illustrated in FIG. 4(B), one shaft 29A may be molded integrally withthe magnet body 25 whereas the other shaft 29B may be a metallic shaftembedded in the magnet body 25. These shafts 26A, 26B, 28A, 28B, 29A,and 29B may be equipped with driving gears for rotating the roller. Whenproducing the magnet rollers shown in FIG. 4(A) and FIG. 4(B), theshafts 28A and 28B or 29B are inserted after forming the magnet body 25with the metal mold.

[0073] The inventors have compared the performance of the magnet roller,which has been manufactured by using the metal mold 20 for magneticfield injection molding shown in FIG. 1 and FIG. 2 and by using themethod in accordance with the present invention, with the performance ofthe magnet roller which has been manufactured by using the metal mold100 for magnetic field injection molding shown in FIG. 14 and by usingthe conventional method.

[0074] As the binder, 12 wt % of nylon 6 was used; and as the magneticpowder, a resin-bonded magnet material which contains 88 wt % of Srferrite was used. The nylon 6 and the resin-bonded magnet material weremelted and mixed by a kneader and the mixture was shaped into pellets byextrusion molding.

[0075] The pellet was injected into the metal mold 20 shown in FIG. 1and FIG. 2 around which a magnetic field generator was provided tomagnetize it in a 4-pole magnetic force pattern composed of S1, N1, S2,and N2. Thus, a magnet roller which measured 17.5 mm in the diameter ofthe magnet body and 304 mm in length. The injecting conditions were asfollows: the cylinder temperature was 280 degrees centigrade, themolding temperature was 100 degrees centigrade, the injection pressurewas 700 kg/cm²; and the back pressure was 7.0 kg/cm².

[0076] Another pellet as stated above was also injected into theconventional metal mold 100 shown in FIG. 14 around which the magneticfield generator was provided to magnetize it in the 4-pole magneticforce pattern composed of S1, N1, S2, and N2. Thus, a magnet roller forthe comparison was made; it measured 17.5 mm in the diameter of themagnet body and 304 mm in length. The same molding conditions wereapplied.

[0077] The results of the measurement of the properties of the magnetroller produced in accordance with the present invention and of theproperties of the magnet roller produced in accordance with theconventional method (Comparative Example 1) are shown in Table 1. Theripple value in the table indicates the maximum difference in magneticforce between adjacent-points when the magnetic force of pole S1 of themagnetic roller is measured at intervals of 1 mm along the axis of theroller. The warp is based on the difference between maximum value andminimum value measured when the magnet roller was rotated with a dialgauge applied to the central part of the magnet body and with the shaftson both ends of the magnet roller supported. The standard deviationvalues of the ripple and warp given in the table are those taken fromthe standard deviation values of the ripples and warps observed in lotsof fifty magnet rollers, each of the lots being manufactured accordingto the respective methods. Rz (μm) indicates the 10-point averageroughness measured according to JIS Standard, namely, JIS B0601-1982.The out-of-roundness indicates the difference between the maximum radiusand the minimum radius observed in terms of a cross sectionperpendicular to the axis of the magnet body. The out-of-roundness iszero in the case of an ideal cylindrical shape; the more distorted theshape, the greater the value of the out-of-roundness.

[0078] Surface roughness Rz of the inner surface of the mold was 1.0 μm.The surface roughness Rz of the magnet roller of the Comparative Example1 could not be determined since a flow mark was formed on the surface.TABLE 1 Standard Standard Deviation Out-of- Ripple Deviation of Round-Value of Ripple Warp Warp Rz ness (Gauss) (Gauss) (μm) (μm) (μm) (μm)Embodiment  3.6 0.2 130  23 1.2  60 Comparative 10.5 5.5 420 161 Un- 150Example 1 measur- able

[0079] As is obvious from Table 1, all the ripple value, warp, surfaceroughness Rz, and out-of-roundness of the magnet roller producedaccording to the present invention are remarkably smaller than those ofthe magnet roller of Comparative Example 1. Moreover, as is obvious fromthe standard deviation values of the ripple and warp, the method inaccordance with the present invention enables the manufacture of magnetrollers which exhibit more homogeneous characteristics than those of themagnet rollers manufactured using the conventional method.

[0080] In order to confirm the effects of the present invention inrelation to the structure of the magnet roller, a metal mold shown inFIG. 8 is prepared, and magnet rollers described below are produced asComparative Examples 2 and 3. A metal mold 40 is used to make a magnetroller made integral with a shaft by molding the roller-shaped magnetbody 25 made of a resin-bonded magnet material and a metallic mandrel 27as one piece as shown in FIG. 17. FIG. 8 shows the cross section of themetal mold for magnetic field injection molding, the metal mold beingclamped; FIG. 9 and FIG. 10 show the cross section of the metal moldwhich is molding the magnet roller.

[0081] The metal mold 40 for magnetic field injection molding isconstructed by the fixed mold counterpart 1, the fixed mold counterpart2, and the movable mold 3 which form the cavity 4. At the time ofclamping, the fixed mold counterpart 1 and the fixed mold counterpart 2can be fixed, holding the metallic mandrel 27 at the center of thehollow 14 of the metal mold. The movable mold 3 is composed of acolumnar object of metal, resin, etc. which has a through hole 3D intowhich the mandrel 27 is inserted; at least one O ring 3E made of rubberor the like is provided on the outer circumferential surface forpreventing the melted resin-bonded magnet material from flowing out. Themovable mold 3 is guided by the mandrel 27 fixed on the fixed moldcounterparts 1 and 2 and it is free to move between one end and theother end of the cavity wall surface segment 4B of the fixed moldcounterparts 1 and 2 by sliding along the mold wall surface segment 4E.

[0082] The magnetic field generator 6 for applying a magnetic field tothe whole or a part of cavity 4 is provided around the fixed moldcounterparts 1 and 2. With a parting line 7 as the boundary, the fixedmold counterpart 1 is mounted on the movable section of an injectionmolding apparatus, not shown, and the fixed mold counterpart 2 isattached to the fixed section of the injection molding apparatus.Although they are not shown, the fixed mold counterparts 1 and 2incorporate cooling pipes for controlling the temperature of the metalmold. The top of the mold counterpart 2 has the injection hole 2A inwhich the nozzle 5 of the injection molding apparatus is fitted forinjecting the resin-bonded magnet material therethrough.

[0083] As shown in FIG. 8, when the metal mold 40 is clamped, themovable mold 3 is brought close to the cavity wall surface segment 4B onthe injection hole 2A side so that the cavity 4 has the minimum volume.Under this condition, the magnetic field generator 6 is actuated toapply a predetermined magnetic field to the hollow 14 of the metal moldand the melted resin-bonded magnet material 10 is injected as indicatedby arrow A inside the cavity wall surface segment 4B of the mold 3through the nozzle 5 as shown in FIG. 9. Flowing pressure F of themelted resin magnetic material 10 is received by the cavity wall surfacesegments 4B of the movable mold 3.

[0084] Continued injection of the melted resin-bonded magnet material 10further increases the flowing pressure due to the injection; in responseto the increase in the flowing pressure, i.e. the increase in the meltedresin-bonded magnet material, the movable mold 3 moves back toward theend of the hollow 14 of the metal mold on the opposite side from theinjection hole 2A, i.e. downward in FIG. 9, by the increased volume ofthe melted resin-bonded magnet material 10 while balancing with thefrictional force between the O ring 3E and the wall surface segment 4Eof the mold counterparts 1 and 2, thus increasing the area of the cavityof the cavity wall surface segment 4A.

[0085] Continuing the injection of the melted resin-bonded magnetmaterial and the reverse movement of the movable mold 3 at the same timeenables the resin-bonded magnet material to be uniformly and denselycharged in the cavity 4 of the metal mold as shown in FIG. 10. Duringthe molding process, the resin-bonded magnet material 10 is magnetizedinto a resin magnet by the applied magnetic field Cooling the fixed moldcounterparts 1 and 2 at a constant rate molds a magnet roller composedof the resin magnet. After predetermined time elapses, the metal mold 40is split to take out the magnet roller made integral with a shaft whichhas been molded as shown in FIG. 17. The thus-produced magnet roller isemployed as the magnet roller of Comparative Example 2.

[0086] Next, the movable mold 3 is taken away from the metal mold 40shown in FIG. 8, and a metallic mandrel 27 is introduced to be fixed atthe center of the hollow 14 formed by the fixed mold counterparts 1 and2. Then, the magnetic field generator 6 is actuated to apply a magneticfield to the hollow 14, thereby similarly producing a magnet roller madeintegral with a shaft shown in FIG. 17. The thus-produced magnet rolleris employed as the magnet roller of Comparative Example 3.

[0087] The properties of the magnet rollers made integral with shafts ofComparative Examples 2 and 3 were measured. The results are summarizedin Table 2 in a similar manner as Table 1. The conditions for producingthe magnet rollers of Comparative Examples 2 and 3 were the same asthose employed in Table 1 except that an axis of 8 mm φ×304 mm was used.TABLE 2 Standard Standard Deviation Ripple Deviation of Out-of- Value ofRipple Warp Warp Rz Roundness (Gauss) (Gauss) (μm) (μm) (μm) (μm)Compara- 4.3 0.1 315 20 1.9 103 tive Example 2 Compara- 4.8 0.6 350 502.2 141 tive Example 3

[0088] As can be seen from Table 2, when the present invention isemployed for a magnet made integral with the shaft, there is an effectof enhancing the characteristics to some extent, but no such remarkableimprovement as that seen in the case of the magnet without a shaft shownin Table 1 was recognized.

[0089] Using the magnet roller, which had been produced according to thepresent invention, a developing roller employed for the developingsection in an electrophotographic copier or laser beam printer wasproduced. A developing roller 50 has a structure shown by thecross-sectional view given in FIG. 5, wherein an aluminum cylinder orsleeve 51 is disposed with a small gap provided between itself and themagnet roller 25 and side discs 52A and 52B made of a nonmagneticmaterial are attached to the ends thereof by fixing sections 54A and54B. The sleeve 51 is rotatably mounted on the shafts 26A and 26B of themagnet roller 25 by bearings 53A and 53B. The shaft 26A of the magnetroller passes through the side disc 52B. The side disc 52A is providedwith a shaft 55.

[0090] The developing roller 50 is mounted on the developing section ofa copier or a laser beam printer by fixing the shaft 26B of the magnetroller 25 and by rotatably mounting the shaft 55 of the side disc. Thesleeve 51 is rotated with respect to the fixed magnet roller 25 to carrythe magnetic toner held on the surface of the sleeve and make it adhereto an electrostatic latent image on a photosensitive drum.

[0091] When this developing roller was used for the development, a goodimage which is free of longitudinal streaks attributable to thevariations in the magnetic force in the axial direction was produced.

[0092] In the metal mold for magnetic field injection molding shown inFIG. 1 and FIG. 2, a coil spring 8 as shown in FIG. 6 may be used inplace of the air cylinder as the means for providing a biasing force tothe movable mold via the sliding rod 3A. In this case, the sliding rod3A connected to the rear end of the movable mold 3 is slidably supportedby a bushing 9 incorporated in the mold counterpart 2 and the end of thesliding rod 3A is supported by the coil spring 8 which provides abiasing force to the movable mold 3 via the sliding rod 3A.

[0093]FIG. 7 is a cross-sectional diagram for illustrating anotherexample of the metal mold for magnetic field injection molding employedin the method in accordance with the present invention. A metal mold 30in this example is provided with a plurality of (two in FIG. 7) movablemold counterparts composed of columnar objects of metal, resin, etc.; itis designed so that movable mold counterparts 38 and 3C move forward orbackward equidistantly in lateral symmetry with respect to the injectionhole 2A. The rear ends of the movable mold counterparts 3B and 3C areconnected to the sliding rods 3A and the ends of the sliding rods 3A aresupported by the coil springs 8.

[0094] When the metal mold 30 composed of the fixed mold counterparts 1and 2 is clamped, the movable mold counterparts 3B and 3C are providedwith biasing forces by the coil springs 8 via the sliding rods 3A in thedirection in which they approach toward each other and the volume of thecavity 4 is the minimum. Under this condition, the magnetic fieldgenerator 6 is actuated to apply a predetermined magnetic field and amelted resin-bonded magnet material is injected into the cavity 4 of themetal mold 30 through the nozzle 5. The flowing pressure of the meltedresin-bonded magnet material is received by the cavity wall surfaces ofthe movable mold counterparts 3B and 3C. Continued injection of themelted resin-bonded magnet material further increases the flowingpressure; in response to the increase in the flowing pressure, i.e. theincrease in the melted resin-bonded magnet material, the movable moldcounterparts 3B and 3C move back in the opposite direction from thedirection of the biasing force, i.e. in the lateral direction in FIG. 7,by the increase in the volume of the melted resin-bonded magnet materialwhile balancing with the biasing forces of the coil springs 8 via thesliding rods 3A.

[0095] Thus, continuing the injection of the melted resin-bonded magnetmaterial and the backward movement of the movable mold counterparts 3Band 3C at the same time enables the resin-bonded magnet material to beuniformly and densely charged in the cavity. During the molding process,the resin-bonded magnet material is magnetized into a resin magnet bythe applied magnetic field. Cooling the mold counterparts 1 and 2 at aconstant rate molds a magnet roller composed of the resin magnet. Aftera predetermined time elapses, the metal mold 30 is split to take out themagnet roller 12 made integral with a shaft which has been molded asshown in FIG. 3.

[0096] In FIG. 7, the coil strings 8 are used as the means for providingthe biasing forces to the movable mold counterparts 3B and 3C via thesliding rods 3A; however, other means such as an air cylinder may beused for providing the biasing force in place of the coil spring.

[0097] According to the present invention, the chances of the defects onthe surface and/or inside of magnet rollers manufactured by the magneticfield injection molding method can be remarkably reduced and theoccurrence of warp in the magnet rollers can also be controlled.Moreover, the use of the magnet roller, which is manufactured by themethod in accordance with the present invention, in a developingapparatus or a cleaning apparatus enables the apparatuses to producegood images.

What is claimed is:
 1. An apparatus for producing a magnet roller,comprising: a fixed mold having a cavity for forming a solid magnetbody; a movable mold disposed in the cavity, which is capable ofincreasing or decreasing a volume of the cavity in accordance with aninjected amount of the melted resin-bonded magnet material composedprimarily of magnetic powder and a binder; and a magnetic fieldgenerator disposed around the fixed mold.
 2. An apparatus for producinga magnet roller according to claim 1, wherein the movable mold moves ina lengthwise direction of the cavity against a biasing force provided bybiasing means extending in the cavity.
 3. An apparatus for producing amagnet roller according to claim 1, wherein the movable mold moves in alengthwise direction of the cavity with a back pressure of 0.5 to 50kg/cm² against a biasing force provided by biasing means extending inthe cavity.
 4. An apparatus for producing a magnet roller according toclaim 1, wherein the movable mold moves in a lengthwise direction of thecavity against a biasing force provided by biasing means extending inthe cavity, the biasing means provided with force by a coil spring or anair cylinder.
 5. A method for producing a magnet roller in which aresin-bonded magnet material, which is composed primarily of magneticpowder and a binder, is injected into a cavity of a metal mold whileapplying a magnetic field thereto, wherein a fixed mold having a cavityfor forming a solid magnet body and a movable mold disposed in thecavity and capable of increasing or decreasing a volume of the cavityare used as the metal mold; and while injecting a melted resin-bondedmagnet material into the cavity, the movable mold is moved such that thevolume of the cavity is increased in accordance with the injected amountof the melted resin-bonded magnet material, and a magnet body moldedwithin the cavity is magnetized by a magnetic field generator disposedaround the fixed mold.
 6. A method for producing a magnet rolleraccording to claim 5, wherein the movable mold moves in a lengthwisedirection of the cavity against a biasing force provided by biasingmeans extending in the cavity.
 7. A method for producing a magnet rolleraccording to claim 5, wherein the movable mold moves in a lengthwisedirection of the cavity with a back pressure of 0.5 to 50 kg/cm² againsta biasing force provided by biasing means extending in the cavity.
 8. Amethod for producing a magnet roller according to claim 5, wherein asurface roughness of the magnet roller is 20 mm or less based onJapanese Industry Standard (JIS) 10-point average roughness scale Rz,and when magnetic force is measured at intervals of 1 mm in a directionparallel to axial direction, a difference in magnetic force betweenadjacent points is 10 gauss or less.